Devices and methods are known in the art for measuring acceleration-dependent signals that are relevant to the safety of passengers in a vehicle involved in a crash or other sudden impact. Such acceleration-dependent signals, for example, relate to the present instantaneous acceleration of the vehicle, an acceleration integral and/or the change in velocity. Throughout the present specification, the term "acceleration" is understood to refer to or include negative acceleration or deceleration, as is experienced by a vehicle in a crash.
Trigger devices are also known that can trigger safety devices such as airbags and seat belt tensioners in response to and dependent on those acceleration-dependent signals. To determine whether a safety device should be triggered, the value of the particular incoming acceleration-dependent signal is compared to a so-called trigger threshold, that is the threshold value for triggering the device. For example, an acceleration sensor provided in a vehicle outputs a signal that is indicative of the instantaneous acceleration (or deceleration) of the vehicle. If this acceleration signal exceeds the threshold value, then the trigger circuit will activate a triggering device to deploy the respective associated safety device. various types and arrangements of triggering devices and comparator circuits are known in the art, and the details thereof are not pertinent and are not limiting as to the present invention.
It is also known that a trigger device can have an adaptable or adjustable trigger threshold value, as can be seen in the German Patent DE 38 16 591. Therein, the trigger threshold value is adapted to certain sudden crash-like situations, dependent on various determined operating parameters of the vehicle, in order to increase the trigger sensitivity of the restraining means. Particularly, the above mentioned German Patent discloses using the change in velocity as the correction value for changing the trigger threshold value. An optimal adaptation, however, can only be achieved for new motor vehicles, because the above mentioned operating parameters have now been found to vary over time as a vehicle ages.
Throughout this specification, the terms "age", "aging" and the like are not limited to chronological age, but rather refer to any factor that degrades or deteriorates a motor vehicle, and particularly the chassis or frame of the motor vehicle, from its new condition. Such factors include, without limitation, the chronological age of the vehicle, the total cumulative miles traveled by the vehicle, the total cumulative operating hours of the vehicle, the degree of vibration to which the vehicle has been subjected while operating (e.g. due to traveling on bumpy roads), the degree of rusting of the vehicle frame or body, etc. For example, a vehicle that has been driven a great total cumulative distance under severe conditions (e.g. bumpy unpaved roads, or a number of small accidents or impacts) may be substantially "aged" and deteriorated from its new condition, regardless of its chronological age. Long term tests have shown that, as a vehicle degrades or deteriorates, i.e. "ages" regardless of the cause, the stiffness of the vehicle frame changes significantly as a result of material fatigue, joint fatigue, rust degradation, and the like, especially for some types of vehicles. It has also been shown by tests or investigations, that the existing safety devices such as airbags were not triggered in some cases of frontal impact accidents involving older vehicles, due to the above mentioned aging or degrading influences.